Is it worth running Linux on my Mac to use the built in AMD graphics card?
Blender usually renders considerably faster on linux, but if you are not sure wether you should install linux or not, it will be easier to just create a linux live usb stick and boot on it. This way you can try linux and render stuff without making any changes to your system. When you are done, you can simply plug out the usb stick and boot on your os. The only downside of a live usb is that your changes won't be saved so you will have to download blender every time you boot on it, wich is really annoying. If you have questions simply comment my answer.